cloud-hypervisor: Recursively run rustfmt

"cargo fmt --all -- check" does not check all the other crates in the
repository so replace this with a find command. In the long term it
might be appropriate to use a cargo workspace to solve this problem.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

.travis.yml

@@ -11,4 +11,4 @@ script:
   - cargo build --release
   - cargo test
   - cargo clippy --all-targets --all-features -- -D warnings
-  - cargo fmt --all -- --check
+  - find . -name "*.rs" | xargs rustfmt --check